"Microsoft recently made a change to the licence agreement saying that a new motherboard is equal to a new computer, hence you need to purchase a new Windows licence. Here is what Microsoft has to say: "An upgrade of the motherboard is considered to result in a 'new personal computer' to which Microsoft OEM operating system software cannot be transferred from another computer. If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created and the license of new operating system software is required." Please note that this does not go for retail copies of Windows, but only for OEM versions.

Most home builders (who actually buy a copy) use the OEM version, it's $100-$150 less than retail. OEM is just a disk and license agreement shrink wrapped together. Retail has a large box and manual (that is apparenly worth $100). Dell and Gateway users don't generally do much mobo swapping because of warranty concerns and are generally less tech inclined. This is directly aimed at hobby builders who overwhelmingly purchase the OEM version and are the most likely users to upgrade often.